Once-through steam generators of the prior art which are used in enhanced oil recovery may include one or more steam-generating circuits at least partially defining a radiant chamber into which heat energy is directed, as is well known in the art. The prior art once-through steam generator may be used for enhanced oil recovery, for example, in a steam-assisted gravity drainage (“SAGD”) application. (Those skilled in the art would be aware of other enhanced oil recovery methods involving the use of steam.) In a SAGD application, as is well known in the art, steam produced by the prior art once-through steam generator is directed into oil-bearing ground to enhance recovery of oil therefrom.
As illustrated in FIG. 1, a once-through steam generator (“OTSG”) 10 of the prior art is included in a system 12 for use in a SAGD application. Feedwater is directed into a steam-generating circuit 14 at an inlet end 16 thereof, as indicated by arrow “A”. A part of the steam-generating circuit 14 is located in a convective module 18. As can be seen in FIG. 1, the steam-generating circuit 14 includes a portion thereof which defines a radiant chamber 19, in which one or more pipes 20 of the steam-generating circuit 14 are exposed to radiant heat from a heat source 22, for generating steam. The system 12 includes a first pipe 24 which is connected to the steam-generating circuit 14 at an outlet end 26 thereof. The steam exits the steam-generating circuit 14 at the outlet end 26 thereof and is directed down the first pipe 24 in the direction indicated by arrow “B”.
Those skilled in the art will appreciate that the OTSG 10 may utilize a variety of sources of heat. For example, the heat utilized may be waste heat from a gas turbine. In that situation, the OTSG 10 includes the convective module 18, but does not include a radiant chamber. It will be understood that the relevant issues arising in the prior art in connection with generating steam by utilizing a radiant chamber also arise in other configurations, regardless of the source of heat. For the purposes hereof, a “heating portion” of the OTSG may refer to a radiant chamber and/or a convective module, as the case may be.
As is well known in the art, in some applications, the wet steam which is produced is sent to a steam separator (not shown in FIG. 1) to remove the water content, and the resulting dry steam is then sent down the well.
As is also well known in the art, the various enhanced oil recovery processes using steam involve directing the steam through pipes positioned in the ground. The in-ground pipes may be positioned in various ways, depending on the process and/or on the characteristics and location of the oil-bearing ground. It will be appreciated by those skilled in the art that many different arrangements of in-ground pipes may be used. For instance, the arrangement shown in FIG. 1 is only one of a variety of possible arrangements of in-ground pipes.
In the arrangement illustrated in FIG. 1, the steam is released from a substantially horizontal part 28 of the first pipe 24, via holes therein (not shown) positioned and sized to achieve a substantially consistent release of steam into oil-bearing ground 30, as indicated by arrows identified as “C” in FIG. 1. The system 12 also includes a second pipe 32 with a substantially horizontal part 34, which also has holes (not shown) in it.
As is well known in the art, the steam which is released into the ground via the holes in the horizontal part 28 of the first pipe 24 heats crude oil in the oil-bearing ground 30, and also condenses, resulting in a mixture of crude oil and water which is collected in the substantially horizontal part 34 (as identified by arrows identified as “D”), entering the horizontal part 34 via the holes therein. The oil and water mixture is pumped in the direction indicated by arrow “E” to a tank and other facilities 36 on the surface for processing, i.e., separation of the crude oil and the water. As will be described, the separation of the oil and the water is incomplete, and in addition, many impurities other than oil typically are accumulated in the water.
As indicated above, SAGD is only one example of an enhanced oil recovery process involving steam. Many other such processes are known. From the foregoing, however, it will be appreciated that steam quality is an important parameter in connection with the profitability of a particular enhanced oil recovery system which includes a once-through steam generator. In the prior art, due to limitations in achieving high steam quality (i.e., greater than 80%), higher steam quantity is required to achieve greater oil flow and revenue which means correspondingly higher energy inputs resulting in lower overall revenue.
As is well known in the art, any impurities in the feedwater to the once-through steam generators exit the steam-generating circuit with the wet steam generated therein, unless the steam generator “runs dry”, in which case, an inner wall surface of the pipe loses water contact and becomes dry. Upon such complete vaporization occurring, the impurities precipitate out onto the inner wall surface, forming a deposit which can significantly adversely affect the performance of the steam-generating circuit. The lack of water is said to constitute a “boiling crisis”, as is well known in the art. As the steam quality increases in the circuit (i.e., toward the output end), the remaining water film thickness around the inner surface of the pipe decreases, and the potential for dryout increases.
A cross-section of a portion of the typical horizontal pipe 20 in a prior art steam-generating circuit 14 is shown in FIG. 2A, and a longitudinal cross-section (taken along line A-A in FIG. 2A) is shown in FIG. 2B. The pipe 20 includes an inner bore 38 defined by an inner surface 40. As can be seen in FIGS. 2A and 2B, a mixture of steam (“S”) and water (“W”) moves through the pipe 20 in the direction indicated by arrow “F” in FIG. 2B. The water W flows in the direction indicated by arrow “F” (i.e., toward the outlet end 26) in an annular film against the inner surface 40, and around the steam S in the center of the bore 38, which is also flowing toward the outlet end. In the prior art pipes, droplets 42 of water tend to become separated from the annular water film W and entrained in the flowing steam S, as is well known in the art.
The feedwater is gradually vaporized, as it moves from the inlet end 16 to the outlet end 26 (FIG. 1). As vaporization progresses, the volume of water decreases, and the concentration of impurities increases accordingly in the remaining water content of the wet steam. Ultimately, if the concentration of impurities becomes sufficiently high, impurities precipitate out to form deposits (not shown) on the inner surface 40 (FIGS. 2A, 2B). The deposits form a thermal barrier on the inner surface 40 and increase the pipe wall temperature, ultimately leading to lower piping material strength. In addition, the deposits can reduce the heat transfer and overall amount of produced wet steam flow.
In FIGS. 1, 2A and 2B, the radiant chamber is horizontal. In this situation, the annular film thickness varies around the inner surface 40 due to gravity effects (FIGS. 2A, 2B). When dryout occurs, it typically occurs at the upper part of the inner wall surface 40 because the water layer is thinner at that point. However, as is well known in the art, the radiant chamber may be positioned vertically, rather than horizontally, and a boiling crisis (pipe surface dry out condition) can also occur in a vertical pipe. The radiant chamber 19 is shown positioned horizontally in FIG. 1 for exemplary purposes only. As is well known in the art, the convective module 18 also may be positioned horizontally or vertically, i.e., oriented for flow of gases therethrough horizontally or vertically. The convective module 18 is shown positioned vertically in FIG. 1 for exemplary purposes only.
In the foregoing discussion, the use of wet steam in the SAGD process is outlined. However, it is also common for the water content of the wet steam to be removed at the outlet end of the steam-generating circuit, so that only dry steam is sent down the well. In this situation as well, higher steam qualities are important, because higher steam qualities result in a lower quantity of high-temperature water that is required to be processed (i.e., removed) within the steam plant, i.e., overall plant economics are improved with smaller recycled water inventories.
From the foregoing, it can be seen that it is important to avoid accumulation of deposits (i.e., due to dry out and known as boiling crises). In horizontal pipe orientations, (e.g., the pipe 20 in FIG. 1), because the annular film thickness decreases as steam quality increases, the film thickness at the upper inner surface may become insufficient to maintain wetness, and dry-out of the upper part of the inner surface is therefore a concern. Accordingly, the known once-through steam generator typically is operated so as to avoid a boiling crisis in its steam-generating circuit(s), i.e., the operating parameters are controlled so as to minimize the risk of a boiling crisis occurring. However, although a boiling crisis can be avoided using this approach, this approach results in generally lower steam quality. For instance, steam quality ratings typically are approximately 80% or less. Such relatively low steam quality means, in effect, that energy inputs into known once-through steam generators are relatively inefficiently utilized.
As is well known in the art, in most applications, steps are taken to substantially purify the feedwater (referred to as “conditioning”) before it is pumped into the circuit at the inlet end thereof, so as to minimize the concentration of impurities that have to be dealt with as the water moves through the circuit. However, in the SAGD application for enhanced oil recovery, the extent of conditioning typically is very limited, in order to limit costs. Therefore, in this type of SAGD application, the feedwater typically has relatively high impurities content, i.e., a content that would be unacceptable for most steam generators operating at 100% saturated or superheated outlet steam.
For example, a typical water quality into an enhanced oil recovery OTSG has 8,000 to 12,000 ppm of total dissolved solids (TDS), trace amounts of free oil (1 ppm), high silica levels (50 ppm), dissolved organics (300 ppm), and elevated hardness (1 ppm). The conductivity of this water is in the range of 10,000 micro siemens/cm and compares to less than 1 micro siemens/cm for a typical OTSG producing 100% saturated or superheated steam. The enhanced oil recovery OTSG is operated with wet steam such that the high levels of impurity are concentrated in the water content of the wet steam and carried through the OTSG.
The preferred flow regime in the piping of the heating region 19 is the annular flow regime described above, because wetted wall conditions ensure that dry out does not occur. In this flow regime, a layer of water (wetness) is positioned on the inner surface 40, and also water droplets are entrained within the steam flowing through a central part of the bore of the pipe.
The entrained droplets are separated from the annular film of water W at a point upstream, identified in FIG. 2B as “U1”. As is well known in the art, the concentration of impurities in the annular film of water W increases as the water W approaches the outlet end 26, due to the generation of steam from the feedwater, as the feedwater is moved from the inlet end 16 to the outlet end 26. The impurities in the water are concentrated as the steam is produced.
It will be appreciated by those skilled in the art that, when the droplet becomes separated from the water film, the droplet has the same concentration of impurities as does the annular film of water W at U1. It will also be appreciated that, as the steam (including the entrained droplets) and the annular water film travel along the pipe, a difference develops between the concentrations in impurities in the water film and in the entrained droplets. This is a result of the variation of evaporation rates between the annular film and the entrained droplets.
Heat from the heat source is transmitted to the pipe, and then through the pipe wall, and (largely via conduction) to the annular water film. In contrast, heat transmitted to the entrained droplets is also transmitted through the annular water film and through the steam. It is understood that the annular water film typically has a much higher rate of vaporization than the entrained droplets because the heat flux to the entrained droplets is much less.
The net effect of the entrained water droplets is to reduce the film thickness, resulting in an increase in the concentrations of impurities in the annular water film, i.e., adjacent to the inner surface 40. In turn, this increases the tendency to reach oversaturation levels, and to form deposits on the inner surface 40. The foregoing is typical of the prior art enhanced oil recovery once-through steam generation systems.
As can be seen in FIG. 2A, where the pipe 20 is horizontal, the annular water film W tends to collect at the bottom side of the pipe 20, to define a film thickness T1, that is substantially thicker than a film thickness T2 of the water film W at the top of the pipe cross-section. This is a result of gravity acting on the annular water film.
In the prior art, and as shown in FIGS. 3A and 3B, the radiant pipes 20 are exposed to non-uniform heat flux around the pipe perimeter 44. In FIG. 3A, the pipes (identified for convenience as 20A, 20B, and 20C) are positioned proximal to a housing 45. (It will be understood that, for clarity of illustration, the annular water films W and the entrained water droplets 42 are deliberately omitted from FIG. 3A.) Inner sides 46 of the outer pipe perimeters 44 are directly subjected to heat energy from the heat source (represented by the arrows “G”), while outer sides 48 of the perimeters 44 are only indirectly subjected to heat from the heat source 22.
The heat to which the outer sides 48 are subjected is heat energy from the heat source 22 which is redirected (i.e., reflected) by the housing 45. The redirected heat energy is schematically represented by arrows “H” in FIG. 3A. It will be understood that the heat flux represented by arrows “G” is substantially greater than the heat flux represented by arrows “H”. As can be seen in FIG. 3B, the heat flux to which the steam and water in the pipe 20 are subjected is unevenly distributed. As a result, the annular film of water W is subjected to different rates of evaporation around the perimeter, resulting in a non-uniform concentration of impurities in the remaining water W. This can lead to impurity oversaturation in some regions, resulting in impurities being deposited.
In the horizontal pipe, the non-uniform film thickness (described above) also results in a concentrating of impurities in the thinner part of the film because the thinner film has less diluting effect, compared to the thicker part of the film at the bottom of the pipe.
Those skilled in the art will appreciate that the parts of the steam-generating circuit illustrated in FIGS. 3A and 3B are positioned at the top of the horizontally-positioned heating region. In other pipes in the steam-generating circuit, located elsewhere relative to the heating portion 19, the uneven distribution of heat has different effects on the water film. For example, in a substantially horizontal heating region with a generally circular portion at least partially defined by the steam-generating circuit, some of the pipes are positioned at the bottom, some are at the sides, and some are located between, relative to the heating region. In such a pipe at the bottom of the heating region, for instance, the top of the pipe will be subjected to the greatest heat flux. As noted above, the thinner part of the annular film is at the top of the pipe, so the uneven distribution of heat flux in this situation exacerbates the issues of dry out and/or concentrations of impurities at the inner surface 40 of the pipe 20. It will be apparent to those skilled in the art that the foregoing applies to any heating region in a prior art OTSG, i.e., whether a radiant chamber or a convective module only.